This invention relates to a process for the production of nitroaromatic compounds from aromatic hydrocarbons using modified clay catalysts. This invention particularly relates to a process for nitration of aromatic hydrocarbons using modified clay catalysts without the use of H2SO4. Thus, this process totally eliminates disposal of salts formed consequent to the neutralization of sulphuric acid.
Nitration has been an active area of industrial chemistry for over a century. Nitration process is used for the production of many large-volume chemicals such as nitrobenzene, nitrotoluenes and nitrochlorobenzenes, and other nitroaromatics. These nitroaromatics are vital intermediates for dyes, pharmaceuticals, perfumes and pesticides.
Nitration of aromatic hydrocarbons is performed classically with a mixture of nitric and sulfuric acids. One of the major disadvantages of this method is formation of by-products of polynitration and also causing environmental pollution during disposal of spent acid.
In the past, several methods were proposed for the nitration of aromatic hydrocarbons. For example, a U.S. Patent (U.S. Pat. No. 3,981,935; Sep. 21, 1976) discloses a two stage continuous process for benzene mononitration with HNO3 and H2SO4. Another Japanese Patent (Jpn. Kokai Tokkyo Koho JP 8224,331; Feb. 8, 1982) gives a method in which chlorobenzene is nitrated with a mixture of HNO3 and H2SO4 at 85-150xc2x0 C. (p:o 61.45:37.8). A European Patent (EP 675,104; Oct. 4, 1995) describes a method for nitration of chlorobenzene at 60-160xc2x0 C. using HNO3 and H2SO4. Another Japanese Patent (Jpn. Kokai Tokkyo Koho JP 05,170,706; Jul. 9, 1993) describes a method for nitration of toluene with HNO3 and H2SO4 at 0-50xc2x0 C. (o:m:p 55:2.3:38.9). However, all these methods have a common disadvantage such as use of hazardous H2SO4, whose disposal poses a significant environmental problem. Although it has been known for some time that benzene and its homologs can be nitrated with HNO3 alone, little or no progress has been made in this direction. The disadvantage in this method is the use of large excess of nitric acid (molar ratio of nitric acid to benzene are 2:1 to 4:1) which increases the possibility of poly-nitro compounds formation and affect the economics of the project.
Several catalytic methods are also known for nitration of aromatic hydrocarbons. Although these methods are practiced in laboratories, none of these methods have feasibility on a commercial basis in terms of economics and other factors. For example, a U.S. Patent (U.S. Pat. No. 4,234,470; Nov. 18, 1980) described a method for nitration of benzene, chlorobenzene and toluene with HNO3 in presence of Nafion catalyst. This method employs expensive Nafion resin whose activity is decreasing on each cycle.
Recently attention has been focused on the development of environmentally friendly solid acid catalysts such as zeolites, sulfated zirconia and Nafion especially in Friedel-Crafts reactions to replace environmentally hazardous chemicals, anhydrous aluminium chloride and sulfuric acid respectively in alkylation and nitration reactions.
The main objective of the present invention is the use of cheaply available modified montmorillonite, a smectite clay as a solid acid catalyst in the nitration of aromatic hydrocarbons. Cation exchange of interstitial cations with transition metal ions boosts the Lewis acidity.
Metal-exchanged montmorillonite catalysts were prepared as described in example 1 and employed in the nitration reactions on aromatic compounds, as described in examples 2-21.
Accordingly the present invention provides:
1. A process for the preparation of nitroaromatic compounds from aromatic hydrocarbons using modified clay catalysts which comprises: nitrating aromatic hydrocarbons using fuming nitric acid in the molar ratio of nitric acid to aromatic hydrocarbon 0.3:1 to 1.2:1 in the presence of metal exchanged clay at 25xc2x0-155xc2x0 C. for 0.25 to 2.0 hrs and recovering corresponding nitroaromatic compounds by conventional methods such as herein described.
2. The clay catalyst used is metal ion exchanged clay.
3. Various metal ions used are selected from Al3+, La3+, Cu2+, Fe3+ and Zn2+.
4. Fuming nitric acid is used as the nitrating agent.
5. Aromatic hydrocarbons used are selected from benzene, chlorobenzene, toluene, o-xylene, m-xylene, p-xylene, anisole and naphthalene.
6. Recovery of nitroarenes is carried out by separating the catalyst by filtration and removing the excess aromatic hydrocarbons by distillation or by rotavapor.
7. A process for the preparation of nitroaromatic compounds from aromatic hydrocarbons using modified clay catalysts substantially as herein described with reference to examples 2 to 21.
The process of the present invention is illustrated with the following examples. However it should not limit the scope of the invention.